The present invention relates to a boat propeller transmission, comprising an input shaft for connection to an output shaft from a drive installation, intended to be connected to an output shaft from a drive installation, a reversing mechanism driven by said input shaft for reversing the rotational direction of a propeller shaft relative to the rotational direction of the input shaft, and a gear set, providing at least two different gear speeds in the same direction between the input shaft and the propeller shaft.
Boat propeller transmissions, e.g. in propeller drives of the type which are steerably and tiltably mounted on the outside of a boat transom and which are drivably coupled to an inboard engine, usually have one reversing transmission for reversing the rotational direction of the propeller shaft. Recently, however, two-speed propeller drives have been developed for primarily achieving more rapid acceleration so that the boat will more rapidly reach the planing position for better fuel economy. In boats with turbo-charged diesel engines, the poor charging capacity of the turbo unit at low engine rpm has been compensated with a displacement compressor, which is mechanically driven by the engine and which is coupled in series with the turbo compressor and supercharges in the low rpm range of the engine, but which is disconnected as soon as the charging capacity of the turbo compressor exceeds the charging capacity of the displacement compressor. In this manner, rapid acceleration is achieved, so that the planing position can be rapidly reached. With a two-speed propeller drive unit with a low gear and a direct gear, the engine in low gear reached earlier the rpm at which the turbo compressor charges efficiently, which provides more rapid acceleration and earlier planing over a single speed transmission. With a two-speed drive unit it is possible in a boat with an engine only turbo-charged, to achieve approximately the same performance as a boat with a single speed drive unit and an engine with both a turbo compressor and a displacement compressor. The cost of manufacturing a propeller drive unit with an extra gear speed is, however, significantly lower than the extra cost for the displacement compressor installation.
In a transmission known by U.S. Pat. No. 5,711,742, a two-speed gear step is placed in series with the reversing mechanism. The two-speed gear step shifts automatically, depending on engine speed and load. At lower rpm and/or high load, the low gear speed is engaged independently of the position of the reversing mechanism, which in practice means that backing will normally occur at low gear speed, since one seldom backs with the engine at high rpm. Since a propeller is designed to provide optimum pulling force when driving forward in the upper rpm range of the engine, and thus provides significantly lower pulling force when backing and at low rpm, poorer backing performance is provided, firstly due to the reversed rotational direction and, secondly, due to the lower rpm.
The purpose of the present invention is to achieve a boat propeller transmission of the type described by way of introduction, which can provide better pulling force when backing than what can be achieved with the above described known transmission.
This is achieved according to the invention by virtue of the fact that the gear set is coupled to a control unit, which is connected to a sensor, which senses the position of a gear selector, and that the control unit is disposed, in the reverse position of the gear selector, to lock the gear set in a high gear position, regardless of the rpm. In this way, it is made sure that backing will always occur in the high-gear speed and thus at higher propeller rpm and thus higher propeller pulling force within the lower rpm range of the engine than when backing with the above described known transmission.